The Big Question with Aline Vidotto, astrophysicist

Aline Vidotto is an assistant professor in Astrophysics at Trinity College Dublin. She studies the interaction of exoplanets with their host star’s wind, and how the interaction can affect planetary habitability. She develops 3D simulations of winds of low-mass stars stars, which permeate entire exoplanetary systems.

What is the biggest question facing your field today?

The biggest question in my field of research today is “How do different stars interact with their exoplanets?”. We want to understand how exoplanets are affected by their host stars.

Nature has surprised us with wide the variety of exoplanets out there: their sizes, masses and orbital locations span a range of parameters not existent in the solar system. The same is true for their host stars: many of them at first sight are similar to our Sun, but if we look closely, we will find host stars that are bigger, smaller, younger and even older than our Sun.

All the diversity of exoplanetary systems out there indicates that the physical interactions taking place in these systems are either currently unknown to us or happens with a different intensity than those in the solar system.

The research my group does in Trinity College Dublin focuses on how stars affect their exoplanets through interactions mediated by “stellar winds”. Just like in the Sun, stars are constantly losing particles, which are then injected into the interplanetary medium. This constant outward stream of particles is what forms the wind of a star. Things get a lot more exciting (and a bit more complex) when we realise that these particles are actually tied to magnetic field lines, whose footpoints start at the surface of the star. So, whenever the wind of a star interacts with an exoplanet, it is not just particles that participate in this interaction, but also the stellar magnetic field that is dragged along.

Why is it significant?

You can now imagine that, because of the variety of stars out there, not all winds will be alike. We believe that the younger the star is, their magnetism and winds are more intense. And, just like humans, as stars age, they become less active, with less intense winds and magnetism. So planets orbiting young stars are actually facing a much harsher stellar wind environment than, let’s say, Earth currently is. The reason why Mars lost its atmosphere in the past is often attributed to the fact that the young Sun had a wind that was more intense in the past. The solar wind then eroded the martian atmosphere with time. The reason why this did not happen to Earth is that luckily Earth has a magnetic shield that prevents the solar wind to access our atmosphere.

Another important factor to consider in the case of exoplanetary systems is the distances exoplanets orbit to their host stars. Most of the planets known today orbit at much closer distance to their host stars than Mercury orbits to the Sun. Closer to the star, stellar winds and radiation are much more intense. Thus, close-in planets also face a much harsher stellar wind environment than that compared to the Earth.

We can now draw several parallels to the solar system. If the harsh solar wind environment at young ages caused Mars to lose its atmosphere, planets orbiting closer to their stars or planets orbiting around younger stars could potentially have their atmospheres eroded. So, understanding how different stars produce different winds and how these winds interact with exoplanets is a crucial piece in the puzzle of planetary habitability. Can life emerge in a planet without atmosphere? If the atmosphere of a planet is eroded, can it regenerate a secondary atmosphere? How likely is that the secondary atmosphere will also be eroded? These are some of the open questions in this exciting field of research.

Where is the answer likely to come from?

I believe the answer to these questions will come from many different fronts. It will come from stellar astronomers, who are trying to understand how stellar winds, magnetism and radiation evolve with age and how these properties vary in different stars. It will come from exoplanetary scientists, who are searching for planets that could resemble Earth. It will come from modellers, theoreticians and observers.

We are living in exciting times! New missions are being launched to study exoplanets and their stars and also to probe physical interactions in exoplanetary systems. Computers are getting faster and much more affordable. New collaborations between scientists are eased through the internet. Conferences and workshops are bustling everywhere, bringing scientists of different countries and of different areas together. I have no doubt that this will all make us quickly move forward into understanding how exoplanets are affected by their host stars.